Stable real-time clock takes just 800 na

The M41TC8025 real-time clock IC is said to take
less power than any alternative device, at just 0.8 µA
with temperature compensation fully operational. The SO-14 packaged
IC uses a 400 kHz I2C interface.

The chips temperature compensation guarantees
accuracy to within 2.5 minutes per year (±5 ppm) over
the operating range of -40 to +85 °C. It has a time-of-day
alarm with interrupt, a fixed-cycle timer-interrupt function, a
time-update interrupt function, and a programmable frequency output
(1 Hz, 1024 Hz and 32768 Hz). (From $2.80 ea/1,000 - samples
available now.)